1. Field of the Invention
The present invention relates to processes for the partial oxidation in a membrane wall gasification reactor of heavy residue bottoms recovered from a slurry hydrocracking process to produce a synthesis gas.
2. Description of Related Art
In a typical refinery, crude oil is initially introduced to an atmospheric distillation column or a crude tower where it is separated into a variety of different components including naphtha boiling in the range 36° C. to 180° C., diesel boiling in the range 180° C. to 370° C., and atmospheric bottoms boiling above 370° C. The atmospheric bottoms residue is further processed in a vacuum distillation column where it is separated into a vacuum gas oil (VGO) boiling in the range 370° C. to 520° C. and a heavy vacuum residue boiling above 520° C. The VGO may be further processed by hydrocracking to produce naphtha and diesel, or by fluid catalytic cracking (FCC) to produce gasoline and cycle oils. The heavy vacuum residue can be treated to remove unwanted impurities or converted into useful hydrocarbon products.
There are three principal types of reactors used in the refining industry: fixed bed, ebullated bed and moving bed. In a fixed bed reactor, catalyst pellets are held in place and do not move with respect to a fixed reference frame. Fixed-bed technologies have considerable problems in treating particularly heavy charges containing high percentages of heteroatoms, metals, and asphaltenes, as these contaminants cause the rapid deactivation of the catalyst and subsequent plugging of the reactor. Multiple fixed-bed reactors connected in series can be used to achieve a relatively high conversion of heavy feedstocks boiling above 370° C., but such design would be costly and, for certain feedstocks, commercially impractical, e.g., catalysts must be replaced every 3 to 4 months.
The ebullated bed reactor was developed to overcome the plugging problems associated with fixed bed reactors as the feedstock becomes heavier and the conversion of vacuum residue increases. In an ebullated bed reactor, the catalyst is fluid, meaning that it will not plug-up as is possible in a fixed bed reactor. The fluid nature of the catalyst also allows for on-line catalyst replacement of a small portion of the bed which results in a high net bed activity.
The moving bed reactor combines the advantage of plug flow in a fixed bed operation and the relatively easy catalyst replacement of an ebullated bed technology. The trickle-flow system allows discontinuous catalyst replacement without interrupting the operation. Operating conditions are more severe than for a usual fixed bed reactor, i.e., the pressure can exceed 200 atmospheres, and the temperature can be in the range of between 400° C. to 427° C. The frequency of catalyst replacement depends on the rate of deactivation. During replacement, catalyst movement is slow compared with the linear velocity of the feed. Catalyst addition and withdrawal are performed via a sluice system at the top and bottom of the reactor. The advantage of the moving bed reactor is that the top layer of the moving bed consists of fresh catalyst. Thus, metals and salts deposited on the top of the bed move downwards with the catalyst and are released at the bottom. The tolerance to metals and other contaminants is therefore much greater than in a fixed bed reactor. With this capability, the moving bed reactor has advantages for hydroprocessing of very heavy feeds, especially when several reactors are combined in series.
The decision to use a particular type of reactor is based on a number of criteria including the type of feedstock, desired conversion percentage, flexibility, run length, product quality, and others. In a refinery, the down-time for replacement or renewal of catalyst must be as short as possible. Further, the economics of the process will generally depend upon the versatility of the system to handle feed streams containing varying amounts of contaminants such as sulfur, nitrogen, metals and/or organometallic compounds, such as those found in VGO, DAO and residues.
Studies have been conducted related to converting heavy vacuum residues boiling at 520° C. and above into light hydrocarbons such as naphtha and diesel. A slurry hydrocracking process that converts heavy vacuum residues in the presence of hydrogen and solid catalyst particles or soluble catalysts has been reported by Zhang et al., “A Review of Slurry-Phase Hydrocracking Heavy Oil Technology”, in Energy & Fuels, 2007, 21 (6), 3057-3062. The slurry hydrocracking technology is based on thermocracking. The process differs from the conventional thermocracking processes since it mixes the feed oil, hydrogen and dispersed unsupported catalysts particles together. It appears that the hydrogen is consumed principally to cap free radicals formed by thermocracking. See Matsumura et al., “Hydrocracking Marlim Vacuum Residue With Natural Limonite. Part 2: Experimental Cracking In A Slurry-Type Continuous Reactor”, Fuel, 2005, 84, 417-421, 420. Heavy residue oil, VGO and low-value refractory pitch streams that normally cannot be economically upgraded or even blended into other products such as fuel oil or synthetic crude oil due to their high viscosity and solids content can be processed by slurry hydrocracking technology.
A slurry hydrocracking process is disclosed in U.S. Pat. No. 5,755,955 in which a heavy hydrocarbon feedstock is mixed with coke-inhibiting additive particles to form a slurry and passed upwardly through a confined vertical hydrocracking zone in the presence of hydrogen and in the absence of an active hydrogenation catalyst. A mixed effluent containing a gaseous phase comprising hydrogen and vaporous hydrocarbons and a liquid phase comprising heavy hydrocarbons and particles of the coke-inhibiting additive is removed from the reactor and separated into gaseous and liquid phases. The liquid phase is fractionated to obtain a bottom stream of pitch containing additive particles and an aromatic heavy gas oil fraction. At least a portion of the pitch stream is recycled back to form part of the feed slurry to the hydrocracking zone. The disclosed process suppresses coke formation and improves yields by controlling the ratio of lower polarity aromatics-to-asphaltenes in the reactor.
The catalysts used in the slurry hydrocracking process can be in the form of powdered natural ore, powdered coal, one or more water-soluble or oil-soluble salts which can contain one or more metals selected from cobalt, molybdenum, nickel, iron, tungsten or manganese. A method for preparing a slurry hydrocracking catalyst is disclosed in U.S. Pat. No. 5,474,977 in which a heteropoly acid and a sulfide or a salt of a metal selected from Group IV through Group VIII are mixed with an asphaltene-containing hydrocarbon feedstock and heated to produce an organometallic compound. The organometallic compound is then converted in the presence of hydrogen to produce the slurry hydrocracking catalyst. Other catalysts suitable for use in the slurry hydrocracking process and methods for their manufacture are known in the art.
During the slurry hydrocracking process described above, the solid heterogeneous catalyst(s) must be recovered and/or removed after their catalytic activity falls below a predetermined efficacy, i.e., when the catalyst is deemed to be spent. One study suggests that the catalysts are single-use because they are deactivated by the high concentrations of sulfurous and nitrogenous compounds as well as the high molecular weight organometallic complexes. Supra, Zhang et al. at 3057. The spent solid catalyst can be contaminated with such compounds such as heavy polynuclear aromatic molecules, sulfur, nitrogen and/or metals. Disposal of the spent solid catalyst as a waste material incurs substantial expense and entails environmental considerations.
Gasification is well known in the art and it is practiced worldwide with application to solids and heavy liquid fossil fuels, including refinery bottoms. The gasification process uses partial oxidation to convert carbonaceous materials, such as coal, petroleum, biofuel, or biomass with oxygen at high temperature, i.e., greater than 800° C., into synthesis gas (“syngas”), steam and electricity. The synthesis gas consisting of carbon monoxide and hydrogen can be burned directly in internal combustion engines, or used in the manufacture of various chemicals, such as methanol via known synthesis processes and to make synthetic fuels via the Fischer-Tropsch process.
The major benefits for a refinery using a heavy residue gasification process are that it can provide a source of hydrogen for hydroprocessing to meet the demand for light hydrocarbon products; it produces electricity and steam for refinery use or for export and sale; it can take advantage of efficient power generation technology as compared to conventional technologies that combust the heavy residue; and it produces lower pollutant emissions as compared to conventional technologies that combust heavy residues as a means of their disposal. Furthermore, the gasification process provides a means for the local disposition of the heavy residues where they are produced, thus avoiding the costs for transportation off-site and/or storage; it also provides the potential for disposal of other refinery waste streams, including hazardous materials. Gasification also provides a potential carbon management tool, i.e., a carbon dioxide capture option can be employed if required by the local regulatory system.
Three principal types of gasifier technologies are moving bed, fluidized bed and entrained-flow systems. Each of the three types can be used with solid fuels, but only the entrained-flow reactor has been demonstrated to efficiently process liquid fuels. In an entrained-flow reactor, the fuel, oxygen and steam are injected at the top of the gasifier through a co-annular burner. The gasification usually takes place in a refractory-lined vessel which operates at a pressure of about 40 bars to 60 bars and a temperature in the range of from 1300° C. to 1700° C.
There are two types of gasifier wall construction: refractory and membrane. The gasifier conventionally uses refractory liners to protect the reactor vessel from corrosive slag, thermal cycling, and elevated temperatures that range from 1400° C. up to 1700° C. The refractory is subjected to the penetration of corrosive components from the generation of the synthesis gas and slag and thus subsequent reactions in which the reactants undergo significant volume changes that result in strength degradation of the refractory materials. The replacement of refractory linings can cost several millions of dollars a year and several weeks of downtime for a given reactor. Up until now, the solution has been the installation of a second or parallel gasifier to provide the necessary continuous operating capability, but the undesirable consequence of this duplication is a significant increase in the capital costs associated with the unit operation.
On the other hand, membrane wall gasifier technology uses a cooling screen protected by a layer of refractory material to provide a surface on which the molten slag solidifies and flows downwardly to the quench zone at the bottom of the reactor. The advantages of the membrane wall reactor include reduced reactor dimensions as compared to other systems; an improved average on-stream time of 90%, as compared to an on-stream time of 50% for a refractory wall reactor; elimination of the need to have a parallel reactor to maintain continuous operation as in the case of refractory wall reactors; and the build-up of a layer of solid and liquid slag that provides self-protection to the water-cooled wall sections.
In a membrane wall gasifier, the build-up of a layer of solidified mineral ash slag on the wall acts as an additional protective surface and insulator to minimize or reduce refractory degradation and heat losses through the wall. Thus the water-cooled reactor design avoids what is termed “hot wall” gasifier operation, which requires the construction of thick multiple-layers of expensive refractories which will remain subject to degradation. In the membrane wall reactor, the slag layer is renewed continuously with the deposit of solids on the relatively cool surface. Further advantages include short start-up/shut down times; lower maintenance costs than the refractory type reactor; and the capability of gasifying feedstocks with high ash content, thereby providing greater flexibility in treating a wider range of coals, petcoke, coal/petcoke blends, biomass co-feed, and liquid feedstocks.
There are two principal types of membrane wall reactor designs that are adapted to process solid feedstocks. One such reactor uses vertical tubes in an up-flow process equipped with several burners for solid fuels, e.g., petcoke. A second solid feedstock reactor uses spiral tubes and down-flow processing for all fuels. For solid fuels, a single burner having a thermal output of about 500 MWt has been developed for commercial use.
In both of these reactors, the flow of pressurized cooling water in the tubes is controlled to cool the refractory and ensure the downward flow of the molten slag. Both systems have demonstrated high utility with solid fuels, but not with liquid fuels.
For production of liquid fuels and petrochemicals, the key parameter is the mole ratio of hydrogen-to-carbon monoxide in the dry synthesis gas. This ratio is usually between 0.85:1 and 1.2:1, depending upon the feedstock characteristics. Thus, additional treatment of the synthesis gas is needed to increase this ratio up to 2:1 for Fischer-Tropsch applications or to convert carbon monoxide to hydrogen through the water-gas shift reaction represented by CO+H2O→CO2+H2. In some cases, part of the synthesis gas is burned together with some off gases in a combined cycle to produce electricity and steam. The overall efficiency of this process is between 44% and 48%.
While gasification processes are well developed and suitable for their intended purposes, its applications in conjunction with other refinery operations have been limited.
It is therefore an object of this invention to provide a process for the disposal of heavy residue bottoms recovered from a slurry hydrocracking process that is economically valuable and environmentally friendly, and that is capable of producing a synthesis gas and/or hydrogen that can be used as a feedstream for other processes in the same refinery.